Condensing oil boiler

Slates

Is it better to run a condensing oil boiler with over sized or undersized radiators ??

davgtrek

I had this very decision about 3 years ago in my own house.

Its always going to be better with oversized or low temp radiators.

you will heat the water to a lower temp in boiler and the rooms will heat up slightly quicker. this all means less oil in the long run which is the expensive part of the heating system.

be very careful who you let size your rads. make sure they are sized for a lower tem water. most sizing charts have the two water temp choices for sizing. one is about 60deg and the other 50deg approx.

theres a lot of conflicting opinion on this exact question.

as an over riding guide its always going to be best to heat the room in the quickest time possible and have decent measures in place to gauge the room temp. ideally thermostats that cut the boiler off when required.

if you have plenty of zoning then thats best or if its just a re-fit of a boiler then its a bit of a waste of boiler technology if there aren't even thermo valves fitted to rads.

p

Carlow52

Slates, the post above suggests that a lower temp, larger rad will heat the room quicker.

This may not be the case so thread with caution on this point.

The other concern here is that if ur water from the boiler is say only 50 degree C ur hot water cylinder will be this temp at best, which is less than the required 60 for legionnaires.

In addition the heat coil in the cylinder will be based on a particular input temp

The condensing in the boiler is related to the return water temp so if the rads are properly balanced then the return temp should be okay.

What I generally do is undersize the boiler and use zoning to make it work harder ie make it condense with normal rads.

The final point is that over sized rads will have more water in them

davgtrek

The condensing part of boiler works in the opposite way to a turbo on a car.
when the boiler has to work harder ie. go above 60 degrees temp the condensing aspect stops and the boiler becomes a standard boiler which defeats the purpose of spending more on it.

there is a balancing act to be achieved to get the return water cooled down enough to gain more heat from boiler at lower temps.

You do need to get the water regularly above 60 degrees in your cylinder to prevent legionairres but I have a 2 year old firebird condensing boiler at home which on its lowest setting gives almost 70 degrees in the tank.

where all this science falls down a bit is that the boiler has a crude enough control knob for its output temp. similar to the one in your fridge.
i keep mine on its lowest setting and it seems quite good on oil consumption.

again I stress that regardless of size rads and type of boiler the best investment will be in zoning ( as much as possible ) and programmable thermostats to control when and how warm the rooms need to be.

Ronnie Whelan

I agree with trek zoning and stat control very inportant to answer your question id oversize